The present invention relates to a method of controlling an anti-skid brake system for a vehicle, for example a trailer or the like, which is equipped with one modulator and two wheel speed sensors for each axle, in which the modulator for controlling the anti-skid. brake system is common to right- and left-wheel brakes for each axle supporting right and left wheels, and the wheel speed sensors are disposed for the right and left wheels, respectively. More particularly, the present invention relates to a method of controlling an anti-skid brake system having one modulator and two wheel speed sensors for each axle in which the determination of whether the wheels are in locking tendency is performed by selecting a lower wheel speed out of the right and left wheel speeds, herein referred to as "Select Low Operation".
Recently, a brake of a trailer is also equipped with an anti-skid brake system (hereinafter sometimes referred to as ABS) which performs an anti-skid brake control to adjust braking pressure of a wheel which is in skid tendency during braking. An ABS for the trailer is shown in FIG. 4 as an example of such a conventional ABS for the trailer.
As shown in FIG. 4, the trailer 1 is equipped with right and left front wheels 4 and 3 supported rotatably to a front axle 2, right and left rear wheels 7 and 6 supported rotatably to a rear axle 5, power chambers 9, 8, 11, and 10 for actuating brakes of wheels which are disposed for the right and left front wheels 4, 3 and the right and left rear wheels 7, 6, respectively, an ABS modulator 12 connected to the power chambers 9, 8, 11, and 10 for controlling a supply of compressed air to the power chambers 9, 8, 11, and 10 and adjusting braking force during ABS operation, a service line 13 connected to a relay valve (not shown) in the ABS modulator 12 for supplying service brake signal pressure for controlling the operation of service brakes for the right and left front wheels 4, 3 and the right and left rear wheels 7, 6, an emergency line 14 connected to an emergency valve (not shown) in the ABS modulator 12 for supplying emergency brake signal pressure for controlling the operation of emergency brakes of the right and left front wheels 4, 3 and the right and left rear wheels 7, 6 when an emergency situation occurs, an air reservoir 15 for reserving the compressed air, wheel speed sensors 17 and 16 disposed for the right and left front wheels 4 and 3 respectively for detecting the wheel speed of the corresponding wheels, and an electronic control unit (hereinafter sometimes referred to as ECU) 18 for receiving each wheel speed signal from the wheel speed sensors 17 and 16 and outputting a control signal for controlling the operation of the ABS modulator 12 into the ABS modulator 12. The numerals 19 and 20 designate couplings for connecting the service line 13 and the emergency line 14 to pipelines at a tractor side not shown, respectively. The numeral 21 designates a coupling for connecting the electronic control unit ECU 18 to a electrical wiring at the tractor side.
The trailer 1 is equipped with the one ABS modulator 12 for the power chambers 9, 8 for the right and left front wheels 4, 3 supported to the front axle 2 and equipped with the two wheel speed sensor 17, 16 for the right and left front wheels 4, 3, respectively (in an example shown in FIG. 4, the ABS modulator 12 is also common to the power chambers 11, 10 for the right and left rear wheels 7, 6).
In the trailer 1 as structured mentioned above, the relay valve in the ABS modulator 12 is switched according to a service brake operational signal passing through the service line 13 from the tractor side, so that the compressed air in the air reservoir 15 is sent to each of the power chambers 9, 8, 11, and 10 so as to operate the service brakes of the right and left front wheels 4, 3 and the right and left rear wheels 7, 6. When an emergency situation occurs, such as when the tractor and the trailer are separated, the emergency valve in the ABS modulator 12 is operated according to an emergency brake operational signal passing through the emergency line 14 so as to operate the emergency brakes of the right and left front wheels 4, 3 and the right and left rear wheels 7, 6 in the stone way.
Further, as the ECU 18 determines that at least either of the right and left front wheels 4, 3 is in locking tendency by the Select Low Operation using the wheel speed signal from the wheel speed sensors 17, 16, the ECU 18 outputs an anti-skid brake control signal to the ABS modulator 12. The anti-skid brake control signal makes ABS modulator 12 to perform an anti-skid brake control for adjusting the braking pressure in the power chambers 9, 8, 11, 10, thereby controlling the braking forces of the right and left front wheels 4, 3 and the right and left rear wheels 7, 6 so as to eliminate the locking tendency.
Such a conventional trailer 1 is equipped with the one ABS modulator 12 and the two wheel speed sensors 17, 16 for the right and left wheels for each axle, the determination of whether the wheels are in locking tendency is performed by selecting the lower wheel speed out of the right and left wheel speeds, that is by the Select Low Operation, thereby performing the anti-skid brake control equally to the right and left wheels. The ABS operation by the Select Low Operation is performed, for example, in accordance with an ABS map shown in FIG. 5. That is, each wheel speed of the right and left wheels is compared with a vehicle speed to judge whether each of the right and left wheels is in a pressure reducing mode, a retaining mode, or a pressure intensifying mode, respectively. As a result of this, when at least either of the right and left wheels is in the pressure reducing mode, the ABS for both right and left wheels sets up the pressure reducing mode even when the other wheel is in any mode. Further when at least either of the right and left wheels is in the retaining mode, the ABS for both right and left wheels sets up the retaining mode even when the other wheel is in any mode but the pressure reducing mode. Furthermore, when at least either of the right and left wheels is in the pressure intensifying mode and he other wheel is in the pressure intensifying mode or a service braking state, the ABS for both right and left wheels is set to the pressure intensifying mode.
Therefore, when the vehicle runs, for example, on a split road where surface friction coefficients (hereinafter sometime referred to as ".mu." simply) on the right and left surfaces of the road are greatly different from each other, the wheel on the lower .mu. surface of the road tends to lock. Therefore, the brakes of the right and left wheels are introduced into the pressure reducing mode for the ABS when the ABS determines the locking tendency of the wheel on the lower .mu. surface and the wheel on the lower .mu. surface is in the pressure reducing mode. Thereby, even when the other wheel on the higher .mu. surface is not in locking tendency, the braking pressure of the other wheel on the higher .mu. surface is also reduced so that the whole braking force of the trailer I becomes insufficient, thereby extending the braking distance of the vehicle.
Furthermore, in the conventional trailer 1, such a problem occurs also when torque values of the right and left wheels are quite different from each other, such as during braking when the trailer 1 turns.